Dyeing with Vat dyes: Part 2

Dyeing with Vat dyes: Part 2


Reduced leuco dye is converted to its sodium salt with NaOH to make it water soluble and develop affinity for cellulosics. NaOH is the popular and universally accepted solubilising agent for leuco vat dyes and is multi-functional in nature, viz. (i) it dissolves leuco dye to its Na-salt, (ii) develops affinity of dye for fibre, (iii) neutralises acidic by-products of Na2S2O4 in bath to maintain pH and (iv) suppresses hardness of water, while use in excess increases negative potential of bath and causes lesser colour yield due to more repulsion between dye and cellulose. To keep dye in its water soluble salt form, sufficient excess of both Na2S2O4 and NaOH must be present in bath. Oxidation of Na2S2O4 by atmospheric oxygen as well as oxygen dissolved in water leads to production of acidic products (Na2SO3, NaHSO3) neutralising a part of alkali, necessitating compensation by adding excess alkali in bath. 2NaHSO3 + 2NaOH = 2Na2SO3 + 2H2O. Also, excess alkali is required to combine CO2 getting into liquor from air, to retain reduced form of dye thus making the use of NaOH around 2-3-times higher over its stoichiometric requirement throughout dyeing. In practice, NaOH is added in bath prior to addition of Na2S2O4 – as reduced dye is very unstable and if NaOH is not available for solubilisation instantly – leuco dye gets oxidised back to the original state. Presence of excess NaOH in bath can be checked by dipping piece of filter paper soaked in phenolphthalein which turns pink if excess NaOH is present in bath.
Dyeing Cellulose has higher affinity for sodium salts of leuco vat dyes necessitating control over the temperature and salt; dye solution is to be added in two instalments while dyeing in jigger. Non-ionic dispersing agents promote diffusion and decrease rate of exhaustion at equilibrium; these are not to be essentially added in bath but can be used to promote levelled dyeing; no other chemicals are to be added during reduction and solubilisation except Na2S2O4 and NaOH. Dyes in combination for composite shades must belong to identical application class, even after that compatibility is to be ensured before application. Control over liquor ratio is necessary to control strike rate to achieve levelled shades. Too higher a liquor ratio will definitely produce brilliant levelled shades by reducing effective dye concentration in bath as well as strike rate, but in expense of high power, water, Na2S2O4 and NaOH consumption; whereas a lower ratio gives a relatively dull shade and chances of unevenness are high due to higher strike rate. The working condition is - therefore - a compromise among all dyeing parameters.

Dyed textile is oxidised to restore original dye structure, to trap dye molecules inside fibre and to develop actual shade. It is essential to impart a thorough wash to dyeings to remove last trace of alkali for proper oxidation. Oxidation may be carried out in open air, H2O2 (50–60°C), sodium perborate (50–60°C) or hypochlorite at room temperature for 15–20 min. Air oxidation is suitable for loose structures but it is time consuming and practicable for smaller production schedule only, whereas compact structures are chemically oxidised. Improper oxidation may lead to faulty shades called patchy dyeing, which is mainly due to inadequate washing of dyed textiles; alkali carried forward by textile does not allow oxidising agents to release enough free oxygen; shades produced in this way lack desired fastness. It is preferred to oxidise dyeings at lowest possible temperature for minimum time; otherwise cotton may get oxidised, especially when hypochlorite is used as an oxidising agent. 
If the dyed textile is not imparted intermediate wash before oxidation or the wash is not adequate for complete removal of Na2S2O4 and NaOH, concentration of H2O2 is to be increased accordingly for oxidation, because a part of it gets wasted to consume Na2S2O4 and NaOH. 

Na2S2O4 + 2NaOH + 3H2O2 → 2Na2SO4 + 4H2O

Some vat black dyes contain –NH2 groups and during reduction, these are changed to –NO2 groups. Only single oxidation is not sufficient for restoration of –NH2 groups. Dyeings are oxidised twice with two different oxidising agents, viz. oxidation with hypochlorite (NaOCl) followed by peroxide treatment to recover original hue and tone. Oxidation with either of Na2Cr2O7 or K2Cr2O7 along with CH3COOH or H2SO4 causes deposition of chromium on dyeings and is not preferred. Over-oxidation results in change of tone and shade. Complete washing out of alkali from dyeings prior to oxidation is essential for quality shades. Indanthron blues, viz. C I Vat Blue 4, 6, 16 develop changed hue and tone when air oxidised in the presence of high alkali concentration forming yellow-green azine derivative; the right hue and tone can be restored by revatting only. Best oxidation is achieved at higher temperature and at around neutral pH for all vat dyes. Treatment of dyeings in dilute acid produces a greenish tone in final shade. Dyes belonging to IK and IW classes possess moderate affinity for fibre resulting moderate diffusion and so must not be rinsed before oxidation as the processes are slow and dye may remain on the fibre surface. In these cases – if possible – dyeings should be hydroextracted and oxidised in air.

A final treatment with soap and soda ash (0.5 g/l each) at boil or only steaming for 10–15 min is necessary for all vat dyed textiles to develop desired bright shade. Only steaming is also efficient but fails to remove superficial dyes. Dye–fibre linkage is established through physical forces. Size of a vat dye molecule is too small compared to the pore size of cotton, implying poor wash fastness but soaping promotes aggregation to the remarkable extent to develop excellent wash fastness. This aggregation is accelerated in presence of salt and/or at lower temperature. Tone of shades is attributed to location of dye in respect of fibre axis and when dye molecules are positioned parallel to fibre axis, the true tone of dyeings appear. This positioning is enhanced by rise in temperature at least for a short period. The function of soaping is thus threefolds: (i) It removes loosely attached superficial dyes. (ii) It develops right tone of shade placing dye crystals perpendicular to the fibre axis. (iii) It helps in the formation of large dye crystals to offer superior fastness. Individual vat dyes differ in rate of crystallisation but the change is more rapid and consistency in shade is achieved at higher temperature that is why soaping is always imparted at boil.

Photochemical degradation:
Few yellow, orange and brown vat dyes show deterioration in strength of dyeings and fading of shade on prolonged exposure to light. Absorbed sunlight, atmospheric oxygen and moisture cause slower decomposition of dye structure; presence of metal in the cellulose or dye may catalyse this fading. The effect is also pronounced when specific dyes are in mixture – one shows catalytic action on another. Dyed rayon shows intense catalytic oxidation than cotton. However, the overall light fastness remains above satisfactory level. 

Practical application: 

Exhaust dyeing 
Dye solution is prepared outside with dye, Na2S2O4 (200–300 g) and NaOH (200–300 g) at required vatting temperature in 8–10 l water, is allowed for 10–15 min for complete reduction and solubilisation. A fresh bath is prepared with excess Na2S2O4 and NaOH in jigger or winch at dyeing temperature. Cellulose in wet state is loaded and one-half of the reduced and solubilised dye solution is added to dyebath. After one complete passage of textile or desired time, left dye solution is added and the material is run in this solution for one more passage. Due to higher affinity of dye, jigger is preferred as dyeing is done in open width and maximum surface is offered against dye molecules. Dyeing is done for two turns (~ 30 min) with half dye at the start of each turn, after which salt (10–20 g/l) is added and dyeing is continued for further 1 hour followed by oxidation and soaping. This method is suitable for light to medium weight fabrics up to around 120 g/m2.

Semi-continuous dyeing 
In pad–exhaust method, a dye solution is prepared with only ultra-disperse or ultra-fine grade dye (remain in well dispersed state) and cellulose is padded with this solution, dried at lower temperature to avoid migration followed by loading in jigger. A reduced and solubilised dyebath is prepared in jigger with excess pad liquors – in case pad liquor is not available, one-tenth of total dye may be added – Na2S2O4 and NaOH. Padded textile is dyed in this bath for desired time after which salt is added. Compact and heavy weight fabrics are best dyed in this method as pressure applied during padding ensures dye penetration and thorough dyeing which otherwise becomes difficult in exhaust method.

Continuous dyeing 
Continuous dyeing of cellulose with vat dye could not achieve commercial success on technical grounds. However, cellulose may be padded with ultradisperse vat dye followed by drying, repad in Na2S2O4 and NaOH solution, re-drying and finally passed through a steamer at 100–105°C for 5–10 min for fixation of dye. Better dyeing results are obtained if rongolite is included in padding liquor and padding with Na2S2O4 and NaOH is omitted.

Defects in dyeings and remedies:

Dark selvedge
This is due to:
(i) thicker selvedge which absorbs more dye and
(ii) in jigger dyeing – selvedge remains exposed to air causing rapid oxidation – partially dries up and absorbs more dye during subsequent passing through bath. Spraying of Na2S2O4 and NaOH solution on selvedge during dyeing reduces this problem by retarding oxidation.

Light spot 
This arises due to:
(i) faulty preparation of fabric, 
(ii) local oil stain adhered on textile during spinning which was not effectively removed during pretreatment, 
(iii) partial degradation of cellulose to oxy- or hydrocellulose during scouring. A mild scouring in post-dyeing stage followed by redyeing reduces this problem.

Oxidation marks/dark patches 
This is the most common fault realised on vat dyed goods. Washing after dyeing should be thorough and should eliminate last trace of alkali. Presence of alkali during oxidation with H2O2 develops these marks as dissociation of H2O2 to release oxygen occurs around a neutral pH. Presence of alkali at places on dyeings forces H2O2 to release perhydroxyl ions, not oxygen. Dyeings with such problems are to be treated with a solution of Na2S2O4 and NaOH for 15–30 min at vatting temperature of dye followed by thorough washing and re-oxidation. Striping out of a part of dye in bath may be compensated accordingly.

Streaky dyeing 
This may be due to: 
(i) reed marks on fabric, 
(ii) variable tension in weft and warp causing prominence of either of these two and 
(iii) faulty mercerisation. Dyes possessing good coverage power are to be used to avoid this problem.

Poor rubbing fastness:

Reasons are mainly:
(i) dyeing in too short liquor causing several dye layers formation on surface of cellulose, 
(ii) storing solubilised dye for prolonged time causing partial oxidation of dye and subsequent surface deposition on cellulose,
(iii) incomplete reduction of dye towards end of dyeing, 
(iv) inefficient soaping after dyeing and (v) dyeing in hard water when metal salts may react with anionic dye and this complex is deposited on dyeings. Most of the described faults can be rectified by treating dyeings with NaOH and Na2S2O4 at the temperature of vatting in presence of levelling agents, but with a chance of striping of a part of dye in bath. Addition of 10–15% of dye of original recipe is required in bath. A higher liquor ratio at all stages of dyeing and proper control over vatting and dyeing temperatures is a solution to most of the faults occurs in vat dyeings.

Stripping and correction of faulty dyeings 
When uneven dyeing occurs or the desired shade is not produced, it is required to correct these through partial or complete stripping. Vat dyes produce very fast shades and complete extraction of dye which is impracticable because of strong affinity of dye for cotton. Dyeings are treated with Na2S2O4 and NaOH at specific temperature of reduction for 30 min for levelling of shade as well as partial stripping. Application of non-ionic surfactant in bath promotes rate of levelling. For complete stripping, method used for partial stripping may be carried out with addition of a cationic surfactant. The latter form complexes with reduced and solubilised anionic dye, loses affinity for cotton and comes out from dyeings. Most of the vat dyes are soluble in hot DMSO and so dyes can be extracted with it from dyeings.
Dyeing with Vat dyes: Part 2 Dyeing with Vat dyes: Part 2 Reviewed by Suraj Gupta on April 26, 2020 Rating: 5

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